There are many properties that need to be considered when developing a contact lens material. Optical clarity, dimensional stability, oxygen permeability, wettability and physiological compatibility are all important parameters in determining the success of any new material. The importance of the surface characteristics of material used in the manufacture of contact lenses should not be underestimated. They govern the interaction of a material with the tear film and any tendency the material may have to become deposited during wear. More importantly, the nature of the material surface can have a vital effect on comfort, wearing time and clarity of vision.
A hydrogel is a hydrated, cross-linked polymeric system that contains water in an equilibrium state. Hydrogels typically are oxygen permeable and biocompatible, making them a preferred material for producing biomedical devices, in particular, contact or intraocular lenses.
Conventional hydrogels are prepared from monomeric mixtures predominantly containing hydrophilic monomers, such as, 2-hydroxyethyl methacrylate or N-vinyl pyrrolidone. U.S. Pat. Nos. 4,495,313, 4,889,664 and 5,039,459 disclose the formation of conventional hydrogels. Oxygen permeability of these conventional hydrogel materials relates to the water content of the materials and is typically below 40 barrers. Although this relatively low level of oxygen permeability is suitable for short-term wear of contact lenses, it is insufficient for long-term wear of contact lenses (e.g. 30 days without removal).
The presence of silicone monomer in a hydrogel formulation is known to increase the oxygen permeability of the resulting hydrogel materials. Silicone hydrogel materials typically have higher oxygen permeabilities than conventional hydrogels. Silicone hydrogels are prepared from hydrogel formulations containing at least one silicone monomer and at least one hydrophilic monomer. Either the silicone monomer or the hydrophilic monomer can function as a crosslinking agent (a crosslinking agent is a monomer having multiple polymerizable functionalities). Alternatively, a separate crosslinking agent can be used. Silicone hydrogel materials are described in U.S. Pat. Nos. 4,954,587; 5,010,141; 5,079,319, 5,115,056; 5,260,000; 5,336,797; 5,358,995; 5,387,632; 5,451,617; and 5,486,579; and WO 96/31792.
In order to achieve good vision, a stable uniform tear film must be supported over the front surface of a contact lens. A lens that does not have good wetting characteristics will result in a rapid break up of the pre-lens tear film and a consequent reduction in vision quality. A stable pre-lens tear film also provides a lubricating effect, allowing comfortable lid movement over the front surface of the lens. Contact lens material surface characteristics also govern biocompatibility. A wettable contact lens material is more likely to allow a continuous tear film between the back surface of the lens and the corneal epithelium, without which no material can be considered as biocompatible. A contact lens surface with poor wettability has greater tendency to attract tear-film deposits. As the tear film dries out due to evaporation between blinking, the dry spots form areas prone to deposit formation, especially protein, and this in turn further reduces surface wettability. The clinical performance of any contact lens material is governed by its ability to form a stable pre and post-lens tear film, which in turn is governed by its wettability.